1. Field of the Invention
The present invention relates to a method for wire-bonding a covered wire.
2. Prior Art
Conventionally, as a method for wire-bonding a covered wire, a method disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2-213146 has been known. This method is disclosed in FIG. 3. A covered wire 1 is comprised of a core wire 1a, which is a conductor, and a covering-film 1b, which consists of a macromolecular resin material with electrical insulating properties and is covered as a coating around the circumference of the core wire 1a. The covered wire 1 is supplied from a wire spool, which is not shown in the Figures, and passed through a capillary 4 via a second clamper 2, which is for holding the wire, and a first clamper 3, which is for cutting the wire. The covered wire 1 thus passing through the capillary 4 is connected to the pad of a semiconductor pellet 5 and the lead 6a of a lead frame 6.
FIG. 3(a) shows a state in which a ball 1c has been formed at the tip of the covered wire 1, and the first clamper 3 and second clamper 2 are opened. Furthermore, a removed portion (exposed portion 1d) has been formed beforehand by a method which will be described below in an area extending for a predetermined distance from the tip of the covered wire 1. As seen from FIGS. 3(a) to 3(b), the capillary 4 is lowered so that the ball 1c is bonded to the first bonding point of a pad of the semiconductor pellet 5. Next, the capillary 4 is raised, moved to a point above one of the leads 6a of the lead frame 6 and then lowered; and as shown in FIG. 3(c), the exposed portion Id is bonded to the second bonding point of the lead 6a.
Next, as shown in FIG. 3(d), the capillary 4 is raised by a distance L.sub.1 from the surface of the lead 6a. This distance L.sub.1 is calculated by use of information concerning the first and second bonding positions and the initial set conditions of the bonding apparatus, etc., as described in Japanese Patent Application Laid-Open No. 2-213146. When the first clamper 3 and the capillary 4 are thus raised by a distance L.sub.1, the first clamper 3 closes and holds the covered wire 1. Next, with the first clamper 3 closed, the clamper 3 is raised together with the capillary 4; and as shown in FIG. 3(e), the covered wire 1 is cut from the base area of the second bonding point. In this case, the first clamper 3 and the capillary 4 are raised so that an intended covering-film removal area 1e comes between the pair of discharge electrodes 7 and 8. As result, the covered wire 1 protrudes from the tip of the capillary 4 by the length L.sub.1, and a part of the exposed portion 1d remains at the tip of the covered wire 1.
Next, as shown in FIG. 3(f), the pair of discharge electrodes 7 and 8 are moved in close to the covered wire 1 from both sides in a non-contact state. A voltage is then applied to the discharge electrodes 7 and 8 so that an electric discharge is caused to take place between the electromagnetic parts 71 and 81 and the core wire 1a with the covering-film 1b in between. As a result of the discharge energy, as shown in FIG. 3(g), a portion of the covering-film 1b located at a predetermined position on the covered wire 1 is removed. In other words, the exposed portion 1d shown in FIG. 3(a) is formed. Next, as shown in FIG. 3(g), the discharge electrodes 7 and 8 are withdrawn in a direction away from the covered wire 1.
Next, as shown by the chain lines, the second clamper 2 is closed, and the first clamper 3 is opened. Afterward, as shown in FIG. 3(h), the first clamper 3 and the capillary 4 are lowered, in relative terms, by a distance L.sub.2 from the state shown in FIG. 3(g). In this case, since the covered wire 1 is held (restrained) by the second clamper 2, the covered wire 1 is brought into the interior of the capillary 4 by a distance L.sub.2, so that the tip of the covered wire 1 protrudes from the tip of the capillary 4 by a tail length L.sub.3. In this case, the tail length L.sub.3 of the tip of the covered wire 1 corresponds to a part of the exposed portion 1d, from which the covering-film 1b has been removed.
Next, as shown in FIG. 3(i), the first clamper 3 is closed, and the second clamper 2 is opened; and the capillary 4 is raised to a ball forming level as shown in FIG. 3(j). Next, as shown by chain lines, the discharge electrode 8 is moved so that the electrode surface 8a for ball formation is positioned directly beneath the tip of the covered wire 1. Then, as shown in FIG. 3(k), a high voltage is applied across the discharge electrode 8 and covered wire 1, thus forming the ball 1c. Next, as shown by the chain lines, the discharge electrode 8 is returned to its original position. Then, the first clamper 3 is opened, and the capillary 4 is positioned above the next bonding point (see FIG. 3(a)). Afterward, the series of operations shown in FIGS. 3(a) through (k) are repeated.
In the prior art described above, as shown in FIG. 3(f), only the covering-film 1b at an intended covering-film removal area is removed which is a part of the wire and has the length L.sub.4 required for the next wiring operation. Accordingly, as seen from FIG. 3(j), the covering-film 1b remains on the portion of the wire that corresponds to the tail length L.sub.3 which protrudes from the tip of the capillary 4 during the formation of the ball 1c. As a result, when as shown in FIG. 3(k) a high voltage is applied to the tail length L.sub.3 so as to form the ball 1, the covering-film 1b is melted and scattered by the discharge, and the tip of the capillary 4 is contaminated. Contaminant matter on the tip of the capillary 4 will adhere to the pressure-welded portions of the ball 1c as shown in FIG. 3(b) and to the pressure-welded portions of the lead as shown in FIG. 3(c), thus lowering the reliability of bonding.